Turbochargers for internal combustion engines can include a wastegate for controlling the portion of the exhaust stream that is directed through the exhaust gas turbine stage of the turbocharger. By varying the position of the wastegate, the level of boost provided by the compressor stage of the turbocharger can be increased or decreased accordingly. Engines including twin turbochargers can utilize wastegate control to balance the level of boost provided by each of the turbochargers.
Under some conditions, turbocharger imbalance can occur when a wastegate associated with one of the turbochargers is set to a different position than the wastegate of the other turbocharger or where one of the compressors enters a surge condition. For example, high engine temperatures and various particulates in the exhaust gas can cause a wastegate to become inoperable, rendering it unresponsive to subsequent commands. When both of the wastegates are concurrently commanded to a set position, the unresponsive wastegate can initiate an oscillating imbalance between the turbochargers, which can degrade the engine or turbochargers.
The inventor herein has recognized that intake systems that utilize a combined intake manifold communicating with twin compressors stages arranged in parallel can pose additional challenges for purposes of management of turbocharger imbalance and wastegate operability. For example, it may be difficult to distinguish the separate flow characteristics of each compressor via a sensor provided at the combined flow. However, use of additional sensors for detecting the position or operability of the wastegates or the separate flow characteristics each turbocharger can provide erroneous feedback if the additional sensors become degraded.
The inventor herein has also recognized that exhaust gas sensors are already utilized in some engine systems to provide air/fuel feedback control to the engine. For example, universal exhaust gas oxygen (UEGO) sensors can be positioned just downstream of each of the turbocharger's exhaust turbine and wastegate passages. While these exhaust gas sensors can be used to measure exhaust gas composition, a compressor side imbalance has very little effect on exhaust gas composition. Instead, the loading on the turbines may be altered between banks as a single compressor goes into surge. The turbocharger having the surging compressor can manifest this surge by an unloading of its turbine and the load on the other turbine can increase proportionally as the non-surging compressor supplements the airflow to the engine. This results in an exhaust temperature difference between the exhaust streams. Thus, in some conditions, the exhaust gas sensors can be used as exhaust temperature sensors, whereby the difference between the temperatures of the exhaust streams can be used as an indication of turbocharger imbalance and compressor surge, enabling mitigating action to be taken to reduce the imbalance. Additionally, wastegate functionality can be diagnosed by examining temperature changes of the exhaust streams after a control command has been issued to the wastegates.
Therefore, as one example, the above issues can be addressed by an engine system for a vehicle, comprising: an internal combustion engine including an air intake system and an exhaust system; a first turbocharger including a first compressor arranged along a first branch of the air intake system, a first turbine arranged along a first branch of the exhaust system, and a first turbine bypass passage include a first wastegate; a second turbocharger including a second compressor arranged along a second branch of the air intake system, a second turbine arranged along a second branch of the exhaust system, and a second turbine bypass passage including a second wastegate; a first exhaust gas sensor arranged along the first branch of the exhaust system downstream of the first turbine and first wastegate; a second exhaust gas sensor arranged along the second branch of the exhaust system downstream of the second turbine and the second wastegate; and a control system configured to command both the first wastegate and the second wastegate to a closed position or an opened position and to indicate one of said wastegates as unresponsive to said command in response to a temperature difference between the first and second branches indicated by the first and second exhaust gas sensors.
In this way, wastegate operability and turbocharger imbalance can be identified via a group of sensors that are already provided with the engine for purposes of air/fuel feedback control and mitigating action can be taken to reduce the imbalance, thereby reducing compressor surge.
As another example, a method of operating an engine system is provided, including an engine having a first cylinder bank and a second cylinder bank operatively coupled to at least two turbochargers via a common intake manifold. The method comprises: during a first mode, varying an amount of fuel that is provided to the first cylinder bank responsive to feedback provided by a first exhaust gas sensor arranged along an exhaust passage of the first cylinder bank and varying an amount of fuel that is provided to the second cylinder bank responsive to feedback provided by a second exhaust gas sensor arranged along an exhaust passage of the second cylinder bank; and during a second mode, adjusting an operating parameter of at least one of the turbochargers responsive to a comparison of a first temperature indication provided by the first exhaust gas sensor and a second temperature indication provided by the second exhaust gas sensor.
In this way, an operating parameter, such as turbine geometry, wastegate position, compressor bypass, etc., of at least one of the turbochargers can be adjusted responsive to an indication of temperature provided by the exhaust gas sensors to reduce turbocharger imbalance and/or compressor surge.